It is well known in the art to utilize a hydraulic servo control mechanism to control the displacement of a variable displacement pump to modulate the drive of a hydraulic load. The hydraulic servo control mechanism usually comprises a manually controlled servo valve which modulates the flow of control fluid to a servo motor which is connected to the variable displacement pump in a manner to vary the displacement thereof. As per Hann U.S. Pat. No. 3,212,263 issued on Oct. 19, 1965, it is also known to utilize a resilient link to yieldably apply a manual input to the servo valve in a manner which prevents an excessive manual force from being applied to the servo valve. The resiliently applied manual input cooperates with a pump swash plate feedback linkage which senses the instantaneous displacement of the variable displacement pump to control flow through the servo valve to maintain the swash plate in a position proportional to the input command. The advantages of such system are fully described and disclosed in the aforesaid Hann patent.
It is further known to utilize an electrical control signal to modulate the flow of control fluid to a servo motor which controls the displacement of the pump. As per Moon U.S. Pat. No. 3,365,886 issued Jan. 30, 1968, an electric override control signal is provided by a centrifugal governor on the output shaft of a variable displacement hydraulic transmission. The speed signal controls a solenoid valve which modulates the flow of control fluid. Such electrically controlled modulation of the control fluid is upstream and separate from the manually controlled displacement control valve.
It is furthermore known to use electrically controlled pilot valves to modulate the flow of control fluid from a charge pump in a manner to axially position a servo valve which controls flow to a swash plate servo motor. Such a system is taught in Knapp et al. U.S. Pat. No. 3,901,031 issued Aug. 26, 1975. This reference teaches electrically sensing the swash plate angle and utilizing an electronic circuit to provide a control signal proportional to such swash plate angle when compared to a reference electrical signal representative of a desired swash plate angle. Solenoid valves are modulated by such control signal to hydraulically position the servo valve. There is no manual input to the servo valve and thus there is no force balancing between the electro-hydraulic input and a manual primary command signal.